Recently, with the proliferation of scanners, the digitization of documents has been showing progress. When a digitized document, e.g., an A4-size document, is stored in a full-color bitmap form, the resultant data becomes about 24 Mbytes with a read resolution of 300 dpi, requiring a large-capacity memory. Such large-volume data is not suitable to be attached to mail and transmitted.
In general, therefore, full-color images are compressed. As a compression method for such data, JPEG is known. JPEG is very effective in compressing natural images such as photographs. When, however, a character portion is JPEG-compressed, image deterioration called mosquito noise occurs. According to a conventional method available, therefore, an input image is segmented into a character region and a photographic region, and the character region is MMR-compressed upon binarization while the photographic region is JPEG-compressed, thereby expressing even a full-color image with a small data size while maintaining the quality of the character region.
This method is wherein at the time of decompression, the white portion of a binary image is expressed by making a JPEG image transparent, and the black portion is expressed by a character in its representative color. Another characteristic feature of this method is that one color is assigned to a unit character. This makes it possible to eliminate variations in character images originally expressed in monochrome due to scanner reading. In addition, using this method for a compression system can realize high-image-quality compression.
In addition, as an enhance technique, for example, a method of further increasing the compression ratio by padding a character region with the color of a portion near the character before JPEG compression has been proposed. A color extraction technique of extracting the representative color of a character in a character region portion is also indispensable, and has been proposed before. This color extraction technique is a technique of inputting a binary image of a character region, the coordinates of the character region, and a color image, and extracting a desired color in a character region portion from the color image.
According to the conventional technique, however, a color-inverted character region portion cannot be specified, and hence the inverted character region is JPEG-compressed, resulting in a deterioration in the inverted character portion.
Recently, a technique of specifying an inverted character region portion has been proposed. This makes it necessary to develop a technique of extracting a color from an inverted character region. When the conventional color extraction technique is used without any modification, the color of an inverted character region of an input image which is blurred by variations due to scanner reading or a compression effect is directly reproduced (extracted).